refers to loss of normal brain tissue which is replaced by scar tissue which does not do the function of brain cells. Brain cells once dead do not normally regenerate and usually when one area of the brain is damaged, over a period of time the adjacent parts of the brain take over the functions lost.
However, in your sister's case, the loss of brain tissue is very severe and one half of the brain has already nearly lost complete function. The other half is also affected. Overall, it is unlikely that her mental functions would improve a great deal at her age.